The usefulness of the phenomenon of magnetostriction in linear distance or position measuring devices is recognized by the prior art; for example, see Redding, U.S. Pat. No. 4,305,283; McCrea et al, U.S. Pat. No. 4,158,964; Krisst, U.S. Pat. No. 4,071,818; Edwards, U.S. Pat. No. 4,028,619; and Tellerman, U.S. Pat. No. 3,898,555. A magnet near or around the magnetostrictive wire marks the location to be measured. Such devices can operate with either mechanical or acoustical/mechanical excitation. When an electrical signal propagating along the wire reaches the area of influence of the magnet an electrical strain is generated. Conversely, when an electrical signal propagating along the wire reaches the area of influence of the magnet an acoustical/mechanical strain is generated. Such linear position detectors are utilized as liquid level detectors. The position of the magnet, and hence the liquid level, is determined as a function of the time required for an acoustical/mechanical disturbance to propagate from one end of the wire through the area of influence of the magnet in the case of mechanical excitation or from the position of the magnet to a sensing apparatus located at one end of the wire in the case of electrical excitation.
An improvement on such devices is disclosed by Dumais U.S. Pat. No. 5,017,867 which includes a reflective termination at the foot of the magnetostrictive wire and measures the difference of the propagation times of a pulse from the magnet position to the foot of the wire and reflected back to the head of the device and of a pulse traveling directly from the magnet to the head. This technique provides twice as much resolution of each measurement since the reflected pulse travels twice as far as the direct pulse for each increment of magnet displacement.
In the field of liquid level detection, it is often useful to simultaneously measure liquid level and measure liquid temperature at one or more locations. Many liquids change volume with temperature. Thus a measurement based upon level alone would not distinguish between cases where the mass of liquid had changed and where the mass of liquid is the same but the volume has changed due to a temperature change. Tellerman, U.S. Pat. No. 4,726,226 has proposed a combined apparatus for simultaneously detecting liquid level using a magnetostrictive position detecting apparatus and detecting temperature at a plurality of positions within the liquid via temperature dependent resistors. Tellerman, U.S. Pat. No. 4,726,226, teaches an encoding technique for transmitting both position and temperature information to a remote site using a single pair transmission line. The resistances of the temperature dependent resistors are measured and these values are used to continuously vary the period of a pulse generator. Position measurements are made at the varying pulse periods of the pulse generator. A composite signal is transmitted on the transmission line in the form of a series of pulses. The time between certain non-consecutive pulses is a measure of the liquid level. The time between groups of pulses corresponds to one of the temperature measurements. The sequence of temperature measurements is known to the apparatus receiving the signal via the transmission line, enabling the pulse period to be translated into temperature.
A similar system was proposed in U.S. Pat. No. 5,050,430 which differs from Tellerman, U.S. Pat. No. 4,726,226 in providing plural liquid level/temperature measurements in sequence. The combined apparatus produces a composite signal for transmission on a two wire transmission line including information regarding the linear displacement measured and the temperature measured by each temperature dependent resistor. The resistance of the temperature dependent resistors is measured in a predetermined sequence employing a sequential switching circuit. The resistance of a first reference resistor having a temperature independent resistance which is less than the lowest expected resistance of the temperature dependent resistors is first measured. Next, the resistance of a second reference resistor having a temperature independent resistance which is greater than the highest expected resistance of the temperature dependent resistors is measured. Then, the resistances of the temperature dependent resistors are measured in a predetermined sequence.
Still another proposal for combining the liquid level and temperature measurements is described in the U.S. Pat. No. 5,253,521 issued Oct. 19, 1993 now U.S. Pat. No. 5,253,521, issued Oct. 19, 1993, entitled METHOD OF TRANSMITTING MEASURED TEMPERATURE AND POSITION PARAMETERS FROM A TRANSDUCER, assigned to the assignee of the present invention and incorporated herein by reference. There a magnetostrictive liquid level probe makes a series of level measurements as well as several temperature measurements at various sites in the probe. To efficiently send the data to a remote control station over a two-wire transmission line, a plurality of frames are sequentially transmitted with alternate frames being assigned to level information and the remainder being assigned to temperature information.
All of the above systems utilize the probe to make measurements of liquid level and temperature and report the results to a remote location where the signals can be interpreted and utilized. They recognize the desirability of limiting the communication media to a two-wire transmission line. In many cases, however, additional information from the storage tank or the surrounding region needs to be sent to the remote location, and the same concerns for efficiency and economy prevail. For example, in the case of double-walled tanks, sensors can be placed in the space between the walls to detect hydrocarbon or water leakage into the space. As another example, sensors outside each tank may detect whether a pump is operating or a valve is closed. In some cases, it is required, in addition to the liquid level sensor, to detect a liquid over-fill condition by an auxiliary set point detector in the tank. It is desirable to monitor the auxiliary sensors, whether internal or external, without the extra expense of additional transmission lines and safety barriers required for such lines.